The present invention relates to electronic circuits and more particularly to electronic circuits using closed-loop control signals to minimize distortion.
Conventional mixers are widely used in a variety of electronic circuitry such as radios, cellular telephones or other devices, requiring the product of two waveform signals. Such mixers typically produce an output signal at a frequency that is the sum, or difference, of the two incoming signal frequencies. This output signal or "mixed" signal undergoes further processing to demodulate the desired data that is typically modulated on one of the signals. Prior art mixers can produce output signals having undesired components including direct current ("DC") caused by a nonlinear response of the mixer. An ideal mixer would form the product of two signals and not have these secondary nonlinear responses. Some applications are less tolerant of accommodating such undesired signal components.
A direct conversion receiver ("DCR") is one such application where undesired signals from the mixer can render the receiver non-functioning. Typically, a DCR uses a balanced mixer that receives a radio frequency ("RF") signal and a local oscillator signal. The local oscillator and RF signal are at the same frequency and therefore the modulation on the RF signal is converted directly to baseband. In the presence of a strong applied RF signal which is off-channel or an undesired interfering signal, the second order distortion component in the nonlinear device which comprises the mixer causes a second harmonic and also creates a DC with an unmodulated RF signal, or a conversion of the modulated signal to baseband with a carrier signal that is amplitude modulated. Since the desired RF signal is also converted to zero IF and the modulation to baseband, this can interfere with the desired signal. This problem is often referred to as a spurious demodulation phenomena or direct detection. The undesired RF signal is directly detected or demodulated through second order distortion, not through mixing action, and thus is demodulated (if signal is strong) regardless of signal frequency.